Yttria-stabilized zirconia membrane stability in fluoride melts for the magnesium SOM process

Thesis (M.S.)--Boston UniversityOne proposed industry method for the direct electrolysis of magnesium oxide for magnesium production is the Solid Oxide Membrane (SOM) process. The SOM process offers an energy efficient, low-cost magnesium production alternative with much lower environmental impact compared to other methods of primary magnesium production. During the SOM process, MgO is dissolved in a molten CaF2-MgF2 flux. A yttria-stabilized zirconia (YSZ) membrane is submerged in the flux, and this membrane acts as an oxygen anion conducting SOM tube. The YSZ membrane separates the cathode and flux from the anode. When an electric potential is applied across the electrolysis cell, magnesium cations travel through the flux and are reduced at a stainless steel cathode. Oxygen anions simultaneously move through the YSZ membrane to a liquid silver anode, where the anions are oxidized. The SOM process has been demonstrated successfully on the laboratory scale, but in order for the SOM process to be commercially viable, electrolysis cells must operate for thousands of hours. The stability of the YSZ membrane limits the operating life of the SOM electrolysis cell. This thesis determines YSZ membrane stability in oxyfluoride fluxes for the SOM process so that membrane degradation can be better understood and controlled. One primary degradation pathway of YSZ in the SOM process has been determined to be yttria depletion out of the YSZ membrane. Yttria concentration profiles in YSZ membranes were determined using x-ray spectroscopy, and the concentration profiles were used to analyzed the depletion process. The yttria depletion mechanism was determined to be chemical diffusion, and the diffusion process was modeled. A method of controlling the yttria depletion process by adding small concentrations of YF3 to the flux is described, modeled, and experimentally proven. An optimal range of YF3 concentrations to add to the flux is determined for increasing YSZ membrane stability. This study investigated the role of flux impurities on YSZ membrane stability. The effect of impurities on YSZ membrane stability had not been studied or described before this work. Impurities tested are common to magnesium ores: calcia, silica, sodium oxide, and sodium peroxide. Any degradation effects due to these impurities were analyzed, and methods to remove the negative effects of impurities were described when possible

Mitigating Electronic Current in Molten Flux for the Magnesium SOM Process

The solid oxide membrane (SOM) process has been used at 1423 K to 1473 K (1150 °C to 1200 °C) to produce magnesium metal by the direct electrolysis of magnesium oxide. MgO is dissolved in a molten MgF[subscript 2]-CaF[subscript 2] ionic flux. An oxygen-ion-conducting membrane, made from yttria-stabilized zirconia (YSZ), separates the cathode and the flux from the anode. During electrolysis, magnesium ions are reduced at the cathode, and Mg[subscript (g)] is bubbled out of the flux into a separate condenser. The flux has a small solubility for magnesium metal which imparts electronic conductivity to the flux. The electronic conductivity decreases the process current efficiency and also degrades the YSZ membrane. Operating the electrolysis cell at low total pressures is shown to be an effective method of reducing the electronic conductivity of the flux. A two steel electrode method for measuring the electronic transference number in the flux was used to quantify the fraction of electronic current in the flux before and after SOM process operation. Potentiodynamic scans, potentiostatic electrolyses, and AC impedance spectroscopy were also used to characterize the SOM process under different operating conditions.National Science Foundation (U.S.) (Grant No. 102663)United States. Dept. of Energy (Grant No. DE-EE0005547

Feasibility of a Supporting‐Salt‐Free Nonaqueous Redox Flow Battery Utilizing Ionic Active Materials

Nonaqueous redox flow batteries (NAqRFBs) are promising devices for grid‐scale energy storage, but high projected prices could limit commercial prospects. One route to reduced prices is to minimize or eliminate the expensive supporting salts typically employed in NAqRFBs. Herein, the feasibility of a flow cell operating in the absence of supporting salt by utilizing ionic active species is demonstrated. These ionic species have high conductivities in acetonitrile (12–19 mS cm−1) and cycle at 20 mA cm−2 with energy efficiencies (>75 %) comparable to those of state‐of‐the‐art NAqRFBs employing high concentrations of supporting salt. A chemistry‐agnostic techno‐economic analysis highlights the possible cost savings of minimizing salt content in a NAqRFB. This work offers the first demonstration of a NAqRFB operating without supporting salt. The associated design principles can guide the development of future active species and could make NAqRFBs competitive with their aqueous counterparts.Salt‐free cell: Decreasing the contribution of salt costs to the total electrolyte cost for nonaqueous redox flow batteries is essential for economic viability. A nonaqueous flow battery utilizing ionic active materials completely removes the need for a supporting salt. The cell cycling performance and area‐specific specific resistance are comparable to those of state‐of‐the‐art nonaqueous flow cells with high salt concentrations.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137469/1/cssc201700028-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137469/2/cssc201700028.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137469/3/cssc201700028_am.pd

Electrochemical engineering of low-cost and high-power redox flow batteries

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references.Grid-scale energy storage has emerged as a key technology for improving sustainability in the electricity generation sector, and redox flow batteries (RFBs) are promising devices to serve this application. Unlike enclosed batteries, RFBs implement soluble redox active species dissolved in liquid electrolytes, which are stored in large tanks. The electrolyte is pumped through an electrochemical reactor where the active species are oxidized or reduced. The size of the reactor determines the power rating, while the tank volume determines the total energy capacity, enabling scalability unique to this architecture. Recent studies have investigated a number of strategies to reduce RFB system cost. One pathway is to lower the electrolyte cost via decreased chemical costs or increased electrolyte energy density. Low-cost active species, such as redox active organic molecules (ROMs) or abundant inorganics, have gained notoriety. Raising cell potential, by identifying active species with more extreme redox potentials or implementing non-aqueous electrolytes, is an effective approach in reducing RFB cost because higher cell potential will reduce both electrolyte and reactor costs. Engineering the electrochemical stack for lower area-specific resistance (ASR) is another strategy towards dropping reactor cost through increased cell power. The plethora of options for reducing RFB prices can be overwhelming. As such, the present work combines techno-economic (TE) modeling, reactor optimization, and new electrolyte design as a toolbox for developing a low-cost RFB prototype. The TE model first predicts RFB system price as a function of reactor performance and electrolyte materials properties, quantifying metrics to achieve desired price targets. With respect to reactor performance, the TE model identifies a range of viable reactor ASRs, and cell performance is verified experimentally. A parallel modeling study, incorporating electrolyte conductivities, Butler-Volmer kinetics, and transport in porous media, calculates cell polarization. With respect to active material and supporting electrolyte properties, the TE model provides bounded design spaces for cost effective RFBs, guiding material development campaigns. Through collaborations with organic chemists and guided materials selection, new RFB electrolytes are generated and validated in performance prototypes. Ultimately, this thesis utilizes TE modeling to guide reactor optimization and materials development cycles, targeting cost-conscious RFB design.by Jarrod D. Milshtein.Ph. D

The Critical Role of Supporting Electrolyte Selection on Flow Battery Cost

© 2017 The Author(s) 2017. Published by ECS. Redox flow batteries (RFBs) are promising devices for grid energy storage, but additional cost reductions are needed to meet the U.S. Department of Energy recommended capital cost of $150 kWh-1 for an installed system. The development of new active species designed to lower cost or improve performance is a promising approach, but these new materials often require compatible electrolytes that optimize stability, solubility, and reaction kinetics. This work quantifies changes in RFB cost performance for different aqueous supporting electrolytes paired with different types of membranes. A techno-economic model is also used to estimate RFB-system costs for the different membrane and supporting salt options considered herein. Beyond the conventional RFB design incorporating small active species and an ion-exchange membrane (IEM), this work also considers size-selective separators as a cost-effective alternative to IEMs. The size selective separator (SSS) concept utilizes nanoporous separators with no functionalization for ion selectivity, and the active species are large enough that they cannot pass through the separator pores. Our analysis finds that SSS or H+-IEM are most promising to achieve cost targets for aqueous RFBs, and supporting electrolyte selection yields cost differences in the$100's kWh-1

Quantifying Mass Transfer Rates in Redox Flow Batteries

© The Author(s) 2017. Engineering the electrochemical reactor of a redox flow battery (RFB) is critical to delivering sufficiently high power densities, as to achieve cost-effective, grid-scale energy storage. Cell-level resistive losses reduce RFB power density and originate from ohmic, kinetic, or mass transfer limitations. Mass transfer losses affect all RFBs and are controlled by the active species concentration, state-of-charge, electrode morphology, flow rate, electrolyte properties, and flow field design. The relationship among flow rate, flow field, and cell performance has been qualitatively investigated in prior experimental studies, but mass transfer coefficients are rarely systematically quantified. To this end, we develop a model describing one-dimensional porous electrode polarization, reducing the mathematical form to just two dimensionless parameters. We then engage a single electrolyte flow cell study, with a model iron chloride electrolyte, to experimentally measure cell polarization as a function of flow field and flow rate. The polarization model is then fit to the experimental data, extracting mass transfer coefficients for four flow fields, three active species concentrations, and five flow rates. The relationships among mass transfer coefficient, flow field, and electrolyte velocity inform engineering design choices for minimizing mass transfer resistance and offer mechanistic insight into transport phenomena in fibrous electrodes

Towards Low Resistance Nonaqueous Redox Flow Batteries

© The Author(s) 2017. Published by ECS. All rights reserved. Nonaqueous redox flow batteries (NAqRFBs) are a promising, but nascent, concept for cost-effective grid-scale energy storage. While most studies report new active molecules and proof-of-concept prototypes, few discuss cell design. The direct translation of aqueous RFB design principles to nonaqueous systems is hampered by a lack of materials-specific knowledge, especially concerning the increased viscosities and decreased conductivities associated with nonaqueous electrolytes. To guide NAqRFB reactor design, recent techno-economic analyses have established an area specific resistance (ASR) target of <5 Ω cm2. Here, we employ a state-of-the-art vanadium flow cell architecture, modified for compatibility with nonaqueous electrolytes, and a model ferrocene-based redox couple to investigate the feasibility of achieving this target ASR. We identify and minimize sources of resistive loss for various active species concentrations, electrolyte compositions, flow rates, separators, and electrode thicknesses via polarization and impedance spectroscopy, culminating in the demonstration of a cell ASR of ca. 1.7 Ω cm2. Further, we validate performance scalability using dynamically similar cells with a ten-fold difference in active areas. This work demonstrates that, with appropriate cell engineering, low resistance nonaqueous reactors can be realized, providing promise for the cost-competitiveness of future NAqRFBs

A symmetric organic-based nonaqueous redox flow battery and its state of charge diagnostics by FTIR

Redox flow batteries have shown outstanding promise for grid-scale energy storage to promote utilization of renewable energy and improve grid stability. Nonaqueous battery systems can potentially achieve high energy density because of their broad voltage window. In this paper, we report a new organic redox-active material for use in a nonaqueous redox flow battery, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) that has high solubility (>2.6 M) in organic solvents. PTIO exhibits electrochemically reversible disproportionation reactions and thus can serve as both anolyte and catholyte redox materials in a symmetric flow cell. The PTIO flow battery has a moderate cell voltage of ∼1.7 V and shows good cyclability under both cyclic voltammetry and flow cell conditions. Moreover, we demonstrate that FTIR can offer accurate estimation of the PTIO concentration in electrolytes and determine the state of charge of the PTIO flow cell, suggesting FTIR as a powerful online battery status sensor. This study is expected to inspire more insights in this under-addressed area of state of charge analysis aiming at operational safety and reliability of flow batteries